13.4 Electricity Generation The large-scale production of electrical energy is possible because of electromagnetic induction. An electric generator is a device that transforms other forms of energy into electrical energy. These other forms of energy including; thermal, gravitational, and kinetic energy, can come in the form of renewable or non-renewable energy sources. In the rivalry between Edison and Tesla, the alternating current system won out, so we will look at the generation of alternating current.

13.4 AC Generator – Single Loop A single loop AC generator uses single loop of conducting wire between an external magnetic field. There are two slip rings connected to different sides of the loop, and two brushes to allow current to be directed out to the external circuit. The slip rings rotate with the loop, while the brushes are stationary and make contact with the slip rings. The spinning force is provided by an external source of energy. For example, falling water turn the blades of a turbine at a hydro-electric power plant.

13.4 AC Generator – Single Loop The direction of rotation for the conducting loop is clockwise. On the left side of the loop, the external force pushes the loop upwards. In order to oppose this motion, according to Lenz’s law, the current must flow such that a downward force would act on the left side of the conductor. Using RHR #3, the current on the left side must be into the page in order to produce this downward force.

13.4 AC Generator – Single Loop On the right side of the loop, the external force pushes the loop downwards. In order to oppose this motion, according to Lenz’s law, the current must flow such that an upward force would act on the right side of the conductor. Using RHR #3, the current on the right side must be out of the page in order to produce this upward force.

13.4 AC Generator Tutorial – Step 1 As the AC generator rotates clockwise, what is the direction of current produced in the external circuit? The current on the right side of the loop heads towards slip ring 2, in contact with brush 2. Since the current leads to the +ve terminal of the galvanometer, the needle indicates +ve current.

13.4 AC Generator Tutorial – Step 2 In addition to the factors discussed throughout Chapter 13, the amount of induced current also depends on the angle of the conductor with respect to the external magnetic field. Induced current is at a maximum when the plane of the loop is parallel to the external magnetic field. As the loop rotates towards 90° of rotation, the amount of current decreases. Once the loop is perpendicular to the external magnetic field, the current reads zero.

13.4 AC Generator Tutorial – Step 3 As the loop rotates from 90° to 180°, current on the right side now enters slip ring 1. This reverses the direction of the current in the external circuit. Since the current leads to the -ve terminal of the galvanometer, the needle indicates -ve current.

13.4 AC Generator Tutorial – Step 4 As the loop rotates from 180° to 270°, the current once again decreases until it reaches zero. At this point, the current once again reverses direction and enters the external circuit at slip ring 2. This process repeats itself (periodic). The readings on the galvanometer are plotted below.

13.4 AC Generator – Coil To increase the amount of current generated we could use a coiled conductor wrapped around a soft-iron armature. This increases the strength of the induced magnetic field. To increase the amount of current we could also increase the rate of rotation or use stronger external magnets. The armature is being rotated by an external source of energy.

13.4 AC Generator – Coil: Step 1 The rotation of the generator is clockwise. As the shaded side of the armature is forced away from the north pole of the external magnet, Lenz’s law opposes the motion. The left side of the armature tries to attract the external magnet as a south pole. Using RHR #2 for coils, the current flows down the front of the coil. The amount of current increases to a maximum until the shaded side of the armature starts approaching the south pole of the external magnet, perpendicular to the external magnets.

13.4 AC Generator – Coil: Step 2 Using Lenz’s law, the shaded side of the armature resists going towards the external south pole by repelling. Using RHR #2 for coils, the current flows up the front of the coil, keeping the shaded side of the armature as a south pole. Once the armature is parallel to the external magnets, the amount of current goes to zero.

13.4 AC Generator – Coil: Step 3 Now the shaded side of the armature spins away from the south pole of the external magnet. Lenz’s law predicts the shaded side of the armature becoming a north pole, resisting the motion of moving away by trying to attract the external south pole. The current now reverses direction, flowing down the front of the coil.

13.4 AC Generator – Coil: Step 4 As the shaded side of the armature moves away from the external south pole, the current again increases to a maximum value in the opposite direction. This occurs until the armature begins to approach the external north pole, once it is perpendicular to the external magnets. The shaded side of the armature remains a north pole as it approaches the external north pole, resisting the motion by causing repulsion. The amount of current approaches zero.

13.4 Homework Questions # 1, 3, 4 p.604